Conventional ultrasound scanners create two-dimensional B-mode images of tissue in which the brightness of a pixel is based on the intensity of the echo return. Alternatively, in a color flow imaging mode, the movement of fluid (e.g., blood) or tissue can be imaged. Measurement of blood flow in the heart and vessels using the Doppler effect is well known. The frequency shift of backscattered ultrasound waves may be used to measure the velocity of the backscatterers from tissue or blood. The Doppler shift may be displayed using different colors to represent speed and direction of flow. In power Doppler imaging, the power contained in the returned Doppler signal is displayed.
Conventional ultrasound transducers transmit a broadband signal centered at a fundamental frequency f.sub.0, which is applied separately to each transducer element making up the transmit aperture by a respective pulser. The pulsers are activated with time delays that produce the desired focusing of the transmit beam at a particular transmit focal position. As the transmit beam propagates through tissue, echoes are created when the ultrasound wave is scattered or reflected off of the boundaries between regions of different density. The transducer array is used to transduce these ultrasound echoes into electrical signals, which are processed to produce an image of the tissue. These ultrasound images are formed from a combination of fundamental (linear) and harmonic (nonlinear) signal components, the latter of which are generated in nonlinear media such as tissue or a blood stream containing contrast agents. With scattering of linear signals, the received signal is a time-shifted, amplitude-scaled version of the transmitted signal. This is not true for acoustic media which propagate ultrasound in a nonlinear manner.
The echoes from a high-level signal transmission will contain both linear and nonlinear signal components. Conventional imaging systems produce waves well above the 200 kPa level at which nonlinear phenomena begin to appear. In fact, pressure fields on the order of several MPa are not unusual.
There are a number of classical acoustic phenomena that are either non-reciprocal or nonlinear. In the non-reciprocal category are phenomena such as: mode conversions from compression waves to shear waves, mode conversion to harmonic motions, total internal reflection along boundaries (exceeding Bruster's angle), and simple refractive bending of ray paths, plus other similar phenomena. In the nonlinear category are a great many things. The generation of second harmonics by free bubbles is well documented, as indicated above. Also, various contrast agents incorporating shell-encased gas or other materials are designed to produce harmonic and/or subharmonic returned echoes.
A great many components of the media should become nonlinear as the wave intensity is increased to the point that the molecular "spring constants" begin to exceed their linear or small signal range. Any nonlinearity in the media should cause a very rich form of harmonic generation, and not just the second harmonic variety. It is well-established that high-intensity ultrasonic waves become progressively more nonlinear as they propagate through tissue. In the theory of nonlinear circuits, the signals are expressed as an infinite sum of signal components. The first term in this sum is the linear term and the higher-order terms represent signals whose spectra are multiple convolutions (in frequency) of the original spectrum. This means that much more spectral energy than only the second harmonic is created. These extra spectral components can coexist with the original band as well as be out of the band.
In certain instances ultrasound images may be improved by suppressing the fundamental and emphasizing the harmonic (nonlinear) signal components. If the transmitted center frequency is at f.sub.0, then tissue/contrast nonlinearities will generate harmonics at kf.sub.0, where k is an integer greater than or equal to 2. Imaging of harmonic signals has been performed by transmitting a narrowband signal at frequency f.sub.0 and receiving at a band centered at frequency 2f.sub.0 (second harmonic) followed by receive signal processing.
A nonlinear imaging system using phase inversion subtraction is disclosed in U.S. Pat. No. 5,632,277 to Chapman et al. First and second ultrasound pulses are transmitted into the specimen being imaged in sequence and the resulting receive signals are summed. The first and second pulses differ in phase by 180.degree.. If the ultrasound waves undergo nonlinear propagation or nonlinear interaction with contrast agents or other nonlinear scattering media, then the returned signal will have both linear and nonlinear components. Upon summation, the linear components will cancel, leaving only the nonlinear components to be imaged.
There is a need for an alternative method of isolating the nonlinear components of the echo signals for use in both non-contrast and contrast harmonic imaging.